The present invention relates to hydrocarbon reservoir management. More specifically the present invention relates to monitoring fluid front movements in hydrocarbon reservoirs.
In a hydrocarbon reservoir, oil is produced through wells, under pressure of gas, water, or compaction. Water may be naturally present in the reservoir displacing the oil to urge it out through the well bores. Often, additional water is injected into the reservoir from injection bore located near the production bore. As oil is extracted from production wells, the water moves through the porous medium of the formation, and the oil-water interface changes shape. If the location of the fluid fronts (especially the oil-water interface) is not monitored during production, it is possible that the well will rapidly start producing a mixture of oil and water. In some cases, it is possible for the well to produce more water than oil. One important challenge for reservoir management is therefore to monitor fluid movements in hydrocarbon reservoirs in view of optimizing their drainage.
Well logs are traditionally the primary source of information used to map the distribution of fluids in hydrocarbon reservoirs. Because of the high electrical resistivity of hydrocarbons compared to formation or injection water, open hole well logs of resistivity are typically used to infer water saturation, the percentage of pore volume occupied by water, a quantity that dramatically changes across the oil-water front. Measurement of fluid pressures is also used to estimate multiphase fluid flow properties (e.g. water and oil mobilities) and, indirectly through numerical simulation of reservoir flow, to assess the location of the oil-water interface.
U.S. Pat. No. 5,467,823 (Babour et al., 1995) addresses long-term monitoring of hydrocarbon reservoirs and discloses the use of permanent downhole sensors for monitoring reservoir pressure, but without specifically disclosing fluid front monitoring.
U.S. Pat. No. 5,642,051 (Babour et al., 1997) discloses the use of permanent downhole electrodes to monitor the position of a hydrocarbon/water interface. The ""823 patent does not specifically address the issue of monitoring the location of the oil-water interface and neither patent discloses any method for interpreting data acquired by the sensors in order to predict the location of the oil-water interface over time.
U.S. Pat. No. 6,061,634 (Belani et al., 2000) disclosed a method for performing a pressure-resistivity inversion for data acquired with a logging tool run into a borehole and not by way of using permanent downhole sensors. In a permanent reservoir monitoring context, U.S. Pat. No. 6,182,013 (Malinverno et al., 2001) disclosed a method for interpreting resistivity and pressure measurements acquired simultaneously during a fall-off test after an injection period and dynamically estimating the location of an oil-water interface. This patent is limited to pressure and DC or AC resistivity measurements. All of the above-mentioned patents suffer of certain shortcomings. Either they do not provide solutions for front monitoring or they relate in isolation to permanent sensor monitoring techniques independent of one another, or, maybe more importantly, they don""t mention how to properly address the sensor selection and installation design issues that are critical ones.
It is thus desirable to provide comprehensive method and apparatus that permit not only to evaluate how the various possible monitoring techniques individually perform in a given monitoring situation, but also that synergistically combine data obtained with these techniques, so as to achieve the best possible efficiency in fluid front tracking.
One embodiment of the present invention provides a method of monitoring a fluid front movement. The method includes: determining at least two techniques for monitoring the fluid front movement; determining a configuration of monitoring sensors, corresponding to the at least two monitoring techniques, from a joint sensitivity study of the at least two techniques; acquiring data with the monitoring sensors; and monitoring the fluid front by joint inverting the data.